Hobie Hanson knows that the sounds coming from across the hall can mean only one thing: 4A has a substitute teacher. And she's sinking fast. Hobie's certain 4B will never be so lucky. Their regular fourth grade teacher, Mr. Star, is as healthy as a horse. The very next day, though, Mr. Star rushes out of the room -- "indisposed," the principal says -- and Hobie and his classmates hear "s-u-b." They are prepared. No stranger could possibly withstand the volley from their vast arsenal of sub sinkers. Except, perhaps, Miss Ivanovitch. It soon becomes clear that it might not be the sub who sinks -- this time, it might just be the students.